Mos comprising substrate potential elevating circuitry for esd protection

ABSTRACT

An integrated circuit ( 25 ) formed at a semiconducting surface of a substrate including a common p-layer ( 38 ) includes functional circuitry ( 24 ) formed on the p-layer ( 38 ) including a plurality of terminals (IN, OUT, I/O) coupled to the functional circuitry ( 24 ). At least one ESD protection cell ( 30 ; in more detail  200 ) is connected to at least one of the plurality of terminals of the functional circuitry ( 24 ). The protection cell includes at least a first Nwell ( 37 ) formed in the p-layer ( 38 ), a p-doped diffusion ( 36 ) within the first Nwell ( 37 ) to form at least one Nwell diode comprising an anode ( 37 ) and a cathode ( 36 ). An NMOS transistor  200  is formed in or on the p-layer ( 38 ) comprising a n+ source ( 43 ), n+ drain ( 44 ) and a channel region comprising a p-region ( 41 ) between the source and drain, and a gate electrode ( 45 ) on a gate dielectric ( 46 ) on the channel region. The terminal of the functional circuit ( 24 , PAD) is coupled to the cathode ( 36 ) of the Nwell diode, and the anode ( 37 ) of the Nwell diode is connected in series with a path from the drain ( 44 ) to the source ( 43 ) of the NMOS transistor ( 200 ).

This invention is in the field of semiconductor integrated circuits, and is more specifically directed to integrated circuits having electrostatic discharge (ESD) protection circuitry and techniques for protecting integrated circuits from damage caused by electrostatic discharge events.

BACKGROUND

Modern high-density integrated circuits are known to be vulnerable to damage from the ESD of a charged body (human or otherwise) as it physically contacts an integrated circuit. ESD damage occurs when the amount of charge exceeds the capability of the conduction path through the integrated circuit. The typical ESD failure mechanisms include thermal runaway resulting in junction shorting, and dielectric breakdown resulting in gate-junction shorting (e.g., in the metal-oxide-semiconductor, or MOS, context).

To avoid damage from ESD, modern integrated circuits may incorporate ESD protection devices at each external terminal. ESD protection devices generally operate by providing a high capacity conduction path, so that the brief but massive ESD charge may be safely conducted away from structures that are not capable of handling the event. In some cases, ESD protection is inherent to the particular terminal, as in the case of a power supply terminal which may provide an extremely large p-n junction capable of conducting the ESD charge. Inputs and outputs, on the other hand, typically have a separate ESD protection device added in parallel to the functional terminal. The ideal ESD protection device turns on quickly in response to an ESD event, with large conduction capability, but remains off and presents no significant load during normal operation.

The high operating voltage and failsafe design constraints have generally been addressed through the use of drain-extended MOS transistors (referred to as DE, DEMOS, or DeNMOS in the case of n-channel devices). A conventional DE transistor has its drain region located within a well of the same conductivity type; for example, in the n-channel case, the n-type drain region is placed within a relatively lightly-doped n-type well. The increased drain-to-substrate junction area provided by the well, along with the reduced dopant concentration at the drain-to-substrate junction, greatly increases the junction breakdown voltage, permitting high voltage operation of the transistor while tolerating voltage excursions at the drain that can occur in the absence of a clamp. DeMOS transistors also enable the use of thinner gate dielectric (e.g. oxide), because the voltage drop across the depletion region of the well reduces the electric field at the drain-side edge of the gate dielectric, and thus reduces the number of channel “hot” carriers that are produced. This reduction in “hot” carrier effects, specifically threshold voltage shift, enables the construction of reliable transistors with extremely thin gate dielectrics. DeMOS devices also present high output impedance, which is especially attractive in using the device in analog circuits. DeMOS transistors are therefore very attractive for use at input/output terminals of modern integrated circuits.

It has been observed, however, that DeMOS devices themselves provide relatively poor inherent ESD protection. One mode of operation for ESD protection structures comprises the elevation of the substrate potential (e.g. to achieve V_(SUB)>1V) during ESD conditions. This substrate bias improves the response of the ESD protection device, while not generally affecting normal operation of the integrated circuit being protected.

In one known substrate elevation embodiment, the ESD cell comprises an NMOS transistor and a RC-triggered pump transistor which is operable to raise the substrate potential of the NMOS transistor during ESD events by forcing current directly into the floating substrate. In another substrate elevation arrangement, the ESD cell comprises an NMOS transistor and an Nwell diode connected to the supply line. When an ESD event strikes this cell, the charging of the supply capacitance forces substrate current from the parasitic pnp embedded in the Nwell diode into the NMOS′ substrate, thus raising the substrate potential.

However, known ESD protection structures have at least one of the below described deficiencies:

-   -   1. relatively large circuit area required;     -   2. fill protection for only a portion (e.g. beginning) of the         ESD pulse;     -   3. lack applicability to fail safe applications (e.g. a direct         DC path to the power supply is not available);     -   4. significant capacitive loading; and     -   5. need for extra biasing circuit or connections.

What is needed is a new ESD protection structure and method of protecting functional circuitry against ESD induced damage that eliminates or reduces the deficiencies listed above.

SUMMARY

This Summary is provided to comply with 37 C.F.R. §1.73, requiring a summary of the invention briefly indicating the nature and substance of the invention. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.

Embodiments of the invention generally comprise an ESD protection cell comprising at least one Nwell diode (and in one embodiment a plurality of series connected Nwell diodes) in series with drain to source path of an NMOS transistor. Embodiments of the invention are generally operable using the Nwell diode's parasitic pnp bipolar action (the Nwell is the base and p-substrate or p-surface layer is the collector of the parasitic pnp) in series with the drain to source path of the NMOS transistor, to receive the collector current and direct the substrate current to provide p-substrate or p-surface layer potential elevation. The pad from the terminal of the functional circuit to be protected (e.g. I/O, dedicated IN or Dedicated OUT) is generally connected to the cathode of the Nwell diode, or to the cathode of the first of the Nwell diodes in the case there are a plurality of series connected Nwell diodes. The NMOS transistor can be a DeNMOS transistor.

During an ESD event the main ESD discharge current is generally conducted by the Nwell diode. Due to transistor action of the parasitic pnp comprising the Nwell diode, a current related to the ESD current will pass into the common p-substrate or p-substrate layer thus raising the local p-substrate or p-substrate layer potential of the ESD cell.

The NMOS transistor can comprise a DeNMOS transistor, wherein the channel region further comprises a second Nwell. In one embodiment the n+ source and p-region are connected to one another and tied to Vss. In this embodiment the gate electrode can be connected to the n+ source and the p-region.

In another embodiment a single contiguous Nwell provides both the Nwell for the Nwell diode and the Nwell for the drain of the NMOS. The ESD cell can further comprise a guard ring surrounding the Nwell diode and the NMOS. In one embodiment the guard ring is closer to the NMOS as compared to the Nwell diode. The Nwell diode can be oriented primarily perpendicular with respect to the path from the drain to the source of the NMOS transistor.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates, at a high level, the construction of integrated circuit into which the embodiments of the invention may be incorporated.

FIG. 2 shows a cross sectional view of the electrical arrangement of an ESD protection structure constructed according to a first embodiment of the invention.

FIG. 3 shows a cross sectional view of the electrical arrangement of another ESD protection structure, constructed according to another embodiment of the invention.

FIG. 4 shows a cross sectional and top view of an electrical arrangement of another ESD protection structure, constructed according to another embodiment of the invention.

FIG. 5A shows a pair of simplified electrical equivalent circuits derived from the ESD structures shown in FIGS. 2-4. The Nwell series diode is shown as D1.

FIG. 5B shows a pair of simplified electrical equivalent circuits derived from the ESD structures shown in FIGS. 2-4 modified to have series connected Nwell diodes (D1 and D2) and the NMOS transistor to be realized using NMOS transistors connected in parallel.

FIG. 6 shows an Nwell diode bias arrangement according to an embodiment of the invention that can be used to reduce leakage.

FIG. 7 shows a plot of the normalized pad current (mA/μm) vs. pad voltage as a function of width (W) achieved by an ESD structure according to an embodiment of the invention.

DETAILED DESCRIPTION

The present invention is described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate the instant invention. Several aspects of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One having ordinary skill in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention. The present invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present invention.

An integrated circuit formed at a semiconducting surface of a substrate comprises a common p-layer, functional circuitry formed on the p-layer having a plurality of terminals coupled to the functional circuitry, and at least one ESD protection cell connected to at least one of the plurality of terminals in parallel with the functional circuitry. The substrate itself can provide the common p-layer, or the p-layer may be provided otherwise, such as in an epitaxial arrangement with an underlying substrate. The protection cell comprises at least a first Nwell formed in the p-layer, a p-doped diffusion within the first Nwell to form at least one Nwell diode comprising an anode and a cathode, wherein the terminal of the functional circuit to be protected is coupled to the cathode of the Nwell diode. An NMOS transistor formed on the p-layer comprises an n+ source, n+ drain and a channel region comprising a p-region between the source and drain, and a gate electrode on a gate dielectric on the channel region. The Nwell diode is connected in series with the path from the drain to the source of the NMOS transistor. In certain embodiments of the invention the NMOS transistor can comprise a DeNMOS transistor. Moreover, the NMOS transistor can comprise a plurality of NMOS connected in parallel.

FIG. 1 illustrates, at a high level, the construction of integrated circuit 25 into which the embodiments of the invention may be incorporated. Integrated circuit 25 includes functional circuitry 24, which is the circuitry that realizes and carries out the desired functionality of integrated circuit 25. The capability of functional circuitry 24 may of course vary widely, for example ranging from a simple buffer or interface, on one hand, to a complex programmable device such as a digital signal processor (DSP). The specific functionality contained within functional circuitry 24 is not of particular importance to embodiments of the invention.

Integrated circuit 25 also includes a number of external terminals, by way of which functional circuitry 24 carries out its function. A few of those external terminals are illustrated in FIG. 1 by way of example. It is to be understood that the number of terminals and their function can also vary widely (up to on the order of several hundred terminals in some cases). In the example of FIG. 1, two terminals I/O operate as common input and output terminals, by way of which functional circuitry 24 can receive incoming signals and can generate outputs, as known in the art. A dedicated input terminal IN is also shown in FIG. 1 for integrated circuit, as is a dedicated output terminal OUT; each of terminals IN, OUT are also connected to functional circuitry 24. Power supply terminal Vdd receives a positive power supply voltage in this example, while ground terminal Vss is provided to receive a reference voltage, such as system ground.

According to the certain embodiments of the invention, integrated circuit 25 includes an instance of ESD protection structure 30 connected to each of its terminals shown in FIG. 1. Each ESD protection structure 30 is connected to its corresponding terminal in parallel with functional circuitry 24. As evident in FIG. 1, ESD protection structures 30 are connected between a terminal of functional circuit 24 and Vss (e.g. ground), thus being in parallel with the functional circuitry 24. As will be evident from the following description, ESD protection structure 30 is well-suited for this wide range of terminal functions, including power supply terminals that are necessarily required to have low input impedance. It is noted that except for protection of power supplies, ESD protection structures 30 receive their high voltage input via connection to a terminal of integrated functional circuitry 24, and not Vdd. This aspect of embodiments of the invention provides applicability to fail-safe connections in which a DC path to the Vdd supply is not available.

Referring now to FIG. 2, the electrical arrangement of ESD protection structure 200, constructed according to a first embodiment of the invention, will now be described. The parasitic bipolar and resistances in the semiconductor regions of interest are also shown, as well as the path of the substrate current (I_(SUB)) that flows from the Nwell diode to the NMOS during an ESD event. In this embodiment, the series Nwell diode is in an Nwell separate from the Nwell used by the NMOS transistor. ESD structure 30 comprises a vertical pnp bipolar transistor 35 comprising p-emitter/cathode 36, Nwell base/anode 37 and p-substrate 38. n+ contact 39 is shown to Nwell 37.

DeNMOS transistor 40 comprises p-well/p-body 41, p+ contact 42 to p-well/p-body 41, n+ source 43, n+ drain 44, n-well 48, gate electrode 45 and gate dielectric 46. A spacer 47 is shown on the sidewall of the gate 45/46. n+ drain 44 is in n-well 48. n+ drain 44 is shown directly connected to n-well 37 through connection to n+ contact 39, such as by a metal layer. STI isolation regions 51 are shown for providing isolation between respective terminals. Other isolation schemes (e.g. dielectric isolation) and processes (e.g. LOCOS) may also be used. The distance between n+ drain 44 and n+ contact 39 is shown as L.

Gate electrode 45 is shown optionally coupled (e.g. using metal) to source 43 and p-body 41 via p+ contact 42. Vss is shown coupled to gate electrode 45 n+/source 43 and p-body 41. However, gate electrode 45 can be generally driven at any potential. For ease of design gate electrode 45 can be grounded as shown. PAD in FIG. 2 is shown connected to p-emitter/cathode 36. As described above, PAD is from the terminal of the functional circuitry to be protected (e.g. I/O, dedicated IN or dedicated OUT).

Referring now to FIG. 3, the electrical arrangement of ESD protection structure 300, constructed according to another embodiment of the invention, will now be described. In this embodiment, the series Nwell diode is embedded in the same Nwell used by the NMOS transistor. Thus, the distance L shown in FIG. 2 becomes zero and n+ drain 44 is also used as the n+ contact for n-well 37. Otherwise ESD structure 300 is analogous to ESD structure 200 shown in FIG. 2.

Referring now to FIG. 4, the electrical arrangement of ESD protection structure 400, constructed according to another embodiment of the invention, will now be described. In this embodiment, an N+/Nwell guard ring 73 is added to enclose a previously described ESD protection structure, such as structures 200 or 300 to eliminate or at least minimize the other p+ substrate connection outside the ESD cell “stealing” substrate current. The top view provided makes clear guard ring 73 encloses the entire perimeter of the Nwell diode and NMOS and thus ensures that essentially all the substrate current generated by the series Nwell diode is collected by the substrate associated with the NMOS.

FIG. 5A shows a pair of electrical equivalent circuits derived from ESD structures 200, 300, or 400. As noted above the emitter/cathode is connected to PAD, where PAD is coupled to the terminal of the functional circuit to be protected (e.g. I/O, dedicated IN or Dedicated OUT). The Nwell series diode or the vertical pnp that comprises the Nwell diode series is shown as D1. The circuit at the right highlights the parasitics (vertical pnp action embedded in D1 and the parasitic substrate resistance RSUB), which are generally fundamental for the operation of embodiments of the invention.

FIG. 5B shows a pair of simplified electrical equivalent circuits derived from the ESD structures shown in FIGS. 2-4 modified to have series connected Nwell diodes (D1 and D2) and the NMOS transistor realized using NMOS transistors connected in parallel. Two or more series connected Nwell diodes generally provide extra voltage drop to accommodate higher voltage applications and extra substrate current due to the plurality of Nwell diodes (hence vertical pnp bipolars) as compared to the single Nwell diode embodiments shown in FIGS. 2-4. In the embodiment with two or more series connected Nwell diodes, one but not all of the diodes can be implemented with “diode connected” NMOS transistors. In this embodiment, better substrate elevation performance is generally provided by having the diode connected to the PAD be an Nwell diode (and thus contribute substrate current).

The parallel connected transistors are helpful particularly when the transistor area is large. Larger perimeter ESD structures (e.g. Wtot from 100 μm to 2000 μm) are generally arranged in multi-finger configurations, i.e. with a plurality of individual protection transistors connected in parallel. This is helpful since it is generally not possible drawing a single transistor with 2000 μm gate length (due to a very large area impact).

In addition, although not shown, the NMOS transistor can comprises a plurality of NMOS transistors in a cascoded arrangement. The cascoded arrangement increases high voltage tolerance of the ESD cell, and can be used in combination with the two or more series connected Nwell diodes described above to accommodate even higher voltage applications.

Certain biasing arrangements can improve performance of ESD cells according to embodiments of the invention. For example, to reduce leakage, as shown in the biasing arrangement 600 shown FIG. 6, the Nwell diode can be biased with Vdd₂>Vdd₁. In this arrangement, Vdd₂>Vdd₁, such as 12 V and 5 V, respectively.

The amount of substrate potential elevation provided by embodiments of the invention generally depends on several design parameters. The orientation of Nwell diode with respect to the drain to source path of the NMOS is one design parameters wherein a better elevation generally results from a primarily perpendicular orientation. A larger elevation is generally achieved for a shorter distance of the Nwell diode to the NMOS, which can be <10 μm and include the embodiment shown in FIG. 3 where the distance is zero. In addition, having a farther connection from p+ emitter/cathode 36 to the NMOS provides a higher Rsub, which then requires lower Isub for a given Vsub target. Moreover, enclosing substantially the entire ESD protection cell within an n+/Nwell guarding (such as shown using N+/Nwell guard ring 73 in FIG. 4) ensures that essentially all the Isub generated will be collected by Rsub.

Embodiments of the invention can generally be applied to all NMOS-based ESD protection structures that can benefit from substrate elevation during ESD events. With regard to circuit implementation, in one embodiment an ESD cell according to an embodiment of the invention comprising a Nwell diode series with the source drain path of an DeNMOS is used in a high-voltage cell. Embodiments of the invention are generally suitable for any fail-safe application (i.e. where there is not a path available to the power supply). Accordingly, the protected circuit can generally be any open-output buffer (single/cascoded NMOS/PMOS to ground), or any supply pad.

Embodiments of the invention provide some significant advantages. For example, significant chip area is saved compared to conventional substrate elevating techniques, estimated as generally being about 40%. Moreover, the substrate current generated by the Nwell diode is generally applied for the entire duration of the ESD pulse. As described above, embodiments of the invention allow fail safe applications (when no DC path to supply available). Moreover, a low-capacitance solution (e.g. <200 pF) is generally provided because the Nwell diode in series “shields” the actual capacitance of the large DeNMOS. Moreover, the comparatively large MOS, such as a DeNMOS can be in grounded gate configuration (such as shown in FIGS. 2 and 3), with no need for any extra biasing circuit. The series Nwell diode also reduces leakage from the NMOS due to the series diode voltage drop.

EXAMPLES

FIG. 7 shows a plot of pad current (mA/μm) vs. pad voltage as a function of width (W) of the NMOS. The pad current provided allows elevation of the substrate potential of generally >1 V during ESD conditions to restore a level of ESD robustness with a generally achievable failure current levels (the current at which thermal runaway is initiated) I_(T2)>2 mA/μm. The data provided demonstrates the scalability (i.e. effectiveness) of embodiments of the invention. Scalability to a large width is generally a significant consideration in designing effective ESD cells.

According to embodiments of the invention, ESD protection is provided by this invention for integrated circuits utilizing NMOS transistors including drain-extended MOS transistors. This protection is provided without requiring the masking or other elimination of isolation structures, such as shallow trench isolation structures, and are also compatible with other advanced technologies such as silicide-clad diffusions. In addition, as described above, the ESD protection structures according to this invention provide improved ESD protection performance over conventional structures, including higher failure current levels It2 substrate. The implementation of these inventive structures is compatible with failsafe requirements, and also may be used in connection with low impedance terminals such as power supply terminals.

Although the exemplary devices described above are configured as n-type MOS transistors, the invention also includes devices that are configured as p-type MOS transistors or combinations of n-type or p-type transistors. One of ordinary skill in the art would understand how to fabricate p-type transistors in accordance with the invention, e.g., by inverting the type of dopants, as compared to that shown in the figures.

The semiconductor substrates may include various elements therein and/or layers thereon. These can include barrier layers, other dielectric layers, device structures, active elements and passive elements including, source regions, drain regions, bit lines, bases, emitters, collectors, conductive lines, conductive vias, etc. Moreover, the invention can be based on a variety of processes including CMOS and BiCMOS.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents.

Although the invention has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”

The Abstract of the Disclosure is provided to comply with 37 C.F.R. § 1.72(b), requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the following claims. 

1. An integrated circuit formed at a semiconducting surface of a substrate comprising a common p-layer, comprising: functional circuitry formed on said p-layer having a plurality of terminals coupled to said functional circuitry; at least one ESD protection cell connected to at least one of said plurality of terminals of said functional circuitry, said protection cell comprising: at least a first Nwell formed in said p-layer, a p-doped diffusion within said first Nwell to form at least one Nwell diode comprising an anode and a cathode; an NMOS transistor formed in or on said p-layer comprising a n+ source, n+ drain and a channel region comprising a p-region between said source and drain, and a gate electrode on a gate dielectric on said channel region, wherein said terminal of said functional circuit is coupled to said cathode of said Nwell diode and said anode of said Nwell diode is connected in series with a path from said drain to said source of said NMOS transistor.
 2. The integrated circuit of claim 1, wherein said NMOS transistor comprises a DeNMOS transistor, wherein said channel region further comprises a second Nwell.
 3. The integrated circuit of claim 1, wherein said n+ source and said p-region are connected to one another and tied to Vss.
 4. The integrated circuit of claim 3, wherein said gate electrode is connected to said n+ source and said p-region.
 5. The integrated circuit of claim 1, wherein a single contiguous Nwell provides both said first Nwell and said second Nwell.
 6. The integrated circuit of claim 1, wherein said ESD cell further comprises a guard ring surrounding both said Nwell diode and said NMOS.
 7. The integrated circuit of claim 6, wherein guard ring is closer to said NMOS as compared to said Nwell diode.
 8. The integrated circuit of claim 6, wherein said guard ring comprises an n-type diffusion.
 9. The integrated circuit of claim 1, wherein said Nwell diode is oriented primarily perpendicular with respect to said path from said drain to said source of said NMOS transistor.
 10. The integrated circuit of claim 1, wherein a first one of the plurality of terminals corresponds to an input or an output terminal of said functional circuit.
 11. The integrated circuit of claim 1, wherein said at least one Nwell diode comprises a plurality of series connected said Nwell diodes connected in series.
 12. A method of protecting an integrated circuit from an electrostatic discharge (ESD) event comprising a functional circuit having at least one terminal and at least one ESD protection cell, said protection cell comprising an NMOS transistor and at least one Nwell diode comprising an anode and a cathode, wherein said terminal of said functional circuit is coupled to said cathode of said Nwell diode and said anode of said Nwell diode is connected in series with a path from a drain to a source of said NMOS transistor, and said terminal connected to said cathode of said protection cell, said functional circuit and said ESD circuit both being in or on a substrate having a common p-layer, comprising: said terminal and said cathode of said Nwell diode receiving an electrostatic discharge during said ESD event; and said Nwell diode generating an ESD discharge current responsive to said ESD event, wherein a current related to said ESD discharge current flows into said p-layer raising a local potential of said p-layer proximate to said protection cell.
 13. The method of claim 12, wherein said raising a local potential of said p-layer proximate to said protection cell is maintained during an entire duration of said ESD event.
 14. The method of claim 12, wherein said NMOS transistor comprises an n+ source, n+ drain and a p-channel region between said n+ source and said n+ drain, said n+ source and said p-channel region both connected to one another and tied to Vss.
 15. The method of claim 14, wherein a gate electrode of said NMOS transistor is connected to said n+ source and said p-channel region.
 16. The method of claim 12, wherein said NMOS transistor comprises a DeNMOS transistor.
 17. The method of claim 12, wherein said at least one Nwell diode comprises a plurality of series connected said Nwell diodes connected in series.
 18. An integrated circuit formed at a semiconducting surface of a substrate comprising a common p-layer, comprising: functional circuitry formed on said p-layer having a plurality of terminals coupled to said functional circuitry; at least one ESD protection cell connected to at least one of said plurality of terminals of said functional circuitry, said protection cell comprising: at least a first Nwell formed in said p-layer, a p-doped diffusion within said first Nwell to form at least one Nwell diode comprising an anode and a cathode; an NMOS transistor formed in or on said p-layer comprising a n+ source, n+ drain and a channel region comprising a p-region between said source and drain, and a gate electrode on a gate dielectric on said channel region, wherein said terminal of said functional circuit is coupled to said cathode of said Nwell diode and said anode of said Nwell diode is connected in series with a path from said drain to said source of said NMOS transistor, further wherein said NMOS transistor comprises a DeNMOS transistor, wherein said channel region further comprises a second Nwell.
 19. The integrated circuit of claim 18, wherein said at least one Nwell diode comprises a plurality of series connected Nwell diodes.
 20. The integrated circuit of claim 18, wherein said ESD cell further comprises a guard ring comprising a n-type diffusion surrounding both said Nwell diode and said NMOS. 